1. Field of the Invention
This invention relates to catalysts for hydrotreating hydrocarbon oil that can be easily activated, and to a method of activating the same.
2. Description of the Prior Art
For the so-called hydrotreatment process (treatment of hydrocarbon oil in the presence of hydrogen to effect hydrogenation, hydrodesulfurization, hydrodenitrification and hydrogenolysis), catalysts have been used which comprise, as an active ingredient, at least one member selected from the group consisting of the metals in Groups VI and VII of the Periodic Table, these metals being supported on an inorganic oxide carrier, such as alumina, silica-alumina and titania. Molybdenum and tungsten are frequently used as the Group VI metal, and cobalt and nickel are often employed as the Group VIII metal.
These metals, usually supported on a carrier in the form of inactive oxide, must be activated before use by presulfiding i.e., for conversion from the oxide to the sulfide form.
This presulfiding is generally effected by charging the catalyst to be activated in a reactor for hydrotreatment of hydrocarbon oil and passing a sulfurizing agent together with hydrogen gas through the catalyst bed. The conditions of this presulfiding vary with the type of intended hydrotreatment process and the kind of sulfurizing agent used. When hydrogen sulfide is employed as the sulfurizing agent, it is diluted with hydrogen gas to a concentration of about 0.5 to 5 volume % and the resulting gaseous mixture is passed at a temperature higher than 180.degree. C. (usually higher than 250.degree. C.) in an amount of 1000 to 3000 liters (at standard temperature and pressure) for 1 liter of catalyst. When carbon disulfide, n-butylmercaptan, dimethyl sulfide or dimethyl disulfide is used, it is diluted before use with light hydrocarbon oil and sulfurization is carried out at a temperatue of 250 to 350.degree. C., under a pressure of 20 to 100 Kg/cm.sup.2, at a liquid space velocity of 0.5 to 2 hr.sup.- 1 and with a hydrogen/oil ratio of 200 to 1000 Nl/l. After finishing this presulfiding of catalyst, feedstock to be treated is fed to the reactor to start the hydrotreatment process.
This presulfiding step, on which successful operation of the succeeding hydrotreatment process depends, must be performed with great care by using proper materials. When a diluent is used, for example, a hydrocarbon oil containing no olefin must be selected, as otherwise the catalyst is poisoned by the polymeric substances formed from the olefins contained. In addition, heavy oil is unsuitable as the diluent because of its poor wetting on the catalyst surface due to its high viscosity. As a result, light hydrocarbon oil has to be used as the diluent, leading to an increase in production cost. Furthermore, the sulfurizing agent must be used in a relatively large amount to prevent the catalyst reduction from being inactivated by the reaction with hydrogen at high temperatures, and hence the weight ratio of sulfurizing agent to hydrogen must be maintained at a proper level throughout the presulfiding process. This preliminary step is rarely automated, and requires unusual and cumbersome operations, imposing a heavy burden on the operators. Thus, how to eliminate this presulfiding step, or how to minimize the cumbersome operations involved, has been a subject of major concern.
A method to meet this demand was recently proposed, which comprises impregnating a supported catalyst of an active metal with a polysulfide represented by the general formula of R--S.sub.n --R' (wherein n is an integer of 3 to 20, and R and R' are each hydrogen atom or an organic group of 1 to 150 carbon atoms), and heat-treating the polysulfide-impregnated catalyst in the absence of hydrogen gas at a temperature of 65.degree. to 275.degree. C. and under a pressure of 0.5 to 70 bar; Japanese Patent Kokai No. 111144 (1986). This method, in which the active metal is sulfurized by the polysulfide contained in the catalyst upon heating, eliminates the use of any sulfurizing agent and a diluent therefor when presulfiding is allowed to proceed inside the reactor, thus simplifying the operation. This method also makes it possible to effect presulfiding outside the reactor and to start the hydrotreatment process immediately after the sulfurized catalyst is charged in the reactor. However, the polysulfide has to be used in the form of a solution in an organic solvent for impregnation, and hence a special contrivance is needed for the use of organic solvents in carrying out the impregnation process.